1. Field of the Invention
The present invention relates to the use of a polyamide molding composition with high melt stiffness for coextrusion with a molding composition based on a high-melting-point polymer. The invention also relates to a process that includes coextruding a multilayer composite from a polyamide molding composition with high melt stiffness and from a molding composition based on a high-melting-point polymer, and also relates to the multilayer composites produced by the process.
2. Description of the Related Art
For the purposes of this invention, high-melting-point polymers (also known as HT polymers) are polymers which can be processed only at high temperatures. If the material is a semicrystalline polymer, the crystallite melting point Tm, measured by means of DSC according to ISO 11357, is at least 255° C. In the case of an amorphous polymer, the glass transition temperature Tg, likewise measured by DSC according to ISO 11 357, is at least 180° C. Extrusion of both types of polymer at a speed sufficient for cost-effectiveness is possible only at temperatures around 300° C. or greater.
When polyamides, such as PA12, are coextruded with HT polymers of this type, various difficulties can arise due to the high extrusion temperatures and to the attendant markedly reduced stiffness of the polyamide melt. An excessive difference in melt viscosities leads to instabilities of the layer boundaries, to poor layer thickness distributions and poor wall thickness distributions, and to unsatisfactory overall quality of the extrudate.
For this reason, attempts to coextrude HT polymers such as ETFE (crystallite melting point Tm about 270° C.; processing temperature from 300 to 340° C.) have focused on reducing the melting point of the HT polymer in order to achieve coextrudability with lower-melting-point polymers, such as PA12. An example of a result of these attempts is provided by soft ETFE (e.g. Neoflon RP7000 from Daikin, Japan) whose crystallite melting point is about 255° C. and whose recommended processing temperature is from 280 to 290° C., or the EFEP type of polymer (e.g. Neoflon RP5000 from Daikin), whose crystallite melting point is about 195° C. and whose processing temperature is from about 240 to 285° C. Within that processing latitude, the stiffness of a melt of an extrusion molding composition based on PA12 is sufficient to achieve coextrudability with soft ETFE and, respectively, EFEP, to give sufficient quality. However, when ETFE is modified performance is impaired, for example reduced barrier action with respect to fuels.
U.S. Pat. No. 7,005,481 discloses a polyamide molding composition and describes the use of compounds having at least two carbonate units for modification of polyamides by condensation, permitting reliable and stable establishment of properties and providing the possibility of undertaking repeated processing of the material modified by condensation, without resultant gelling or inhomogeneity. Brüggemann KG markets the product Brüggolen M1251, which is an additive based on this principle for molecular weight adjustment of polyamides. Main applications are in the viscosity adjustment sector for recycled material composed of PA6 or PA66, this material being recycled in molding compositions for extrusion. The additive Brüggolen M1251 is a masterbatch of a low-viscosity polycarbonate, such as Lexan 141, in an acid-terminated PA6. A reaction between the polycarbonate and the amino end groups present in the material to be modified by condensation is the cause of the increase in molecular weight.
Studies have shown that it is also difficult to achieve coextrusion with other high-melting-point polymers, e.g. PPS, for the same reasons described above.